1. Field of the Invention
This invention relates to engraving and, more particularly, to the art of engraving desired geometric patterns on a surface of a cylinder.
2. Description of Related Art
A helical gravure engraver creates periodic ink-receiving cavities, cells or areas on a printing cylinder by rotating the cylinder about its cylindrical axis while moving an engraving head in a direction parallel to the cylinder axis. Engraved areas are engraved on the surface of the cylinder by oscillating an engraving device, such as a diamond stylus, into engraving contact with the cylinder. A cell or cavity is engraved each time the stylus is oscillated into contact with the printing cylinder. The resulting cavities are arranged along a continuous helical track or in adjacent cylindrical tracks.
The stylus may be mounted on the engraving head as generally described in Buechler, U.S. Pat. No. 4,450,486 and may be controlled as described in U.S. Pat. Nos. 5,424,845 and 5,438,422, respectively. As explained in U.S. Pat. Nos. 5,424,845 and 5,438,422, the stylus is oscillated at a frequency having an odd number of half wavelengths during a full engraving rotation. This causes a staggering of engraved areas engraved on successive rotations of the cylinder. The engraving head is advanced an axial distance equal to one-half of a black cell width plus one-half of a connecting channel width, plus one separating wall width during each complete rotation of the printing cylinder. This causes a nesting of cavities which are engraved during successive rotations.
As also taught by U.S. Pat. Nos. 5,424,845 and 5,438,422, the engraver engraves cavities having a size which corresponds to a grey level to be printed by the printing cylinder. The cavity depth is controlled by a stylus driver acting in response to the combined levels of a DC video signal, a white offset signal, and an AC signal. The DC gain and the AC gain may be controlled by a set-up circuit in accordance with a series of set-up parameters.
While the above-described prior art system faithfully engraves cells at a regular frequency which will print a desired grey level, there is a problem with edge definition. This is due to the fact that geometrical patterns are represented by nested clusters of cells. This invention provides an apparatus and method for altering the relative position between two or more engraved areas which, in turn, facilitates providing patterns having, for example, improved edge or line definition.